Various printing plates that use silicone rubber or fluorine resin as ink repellent layer and that are designed for conducting planographic printing without using a dampening solution (hereinafter called waterless planographic printing) have been proposed. Waterless planographic printing is a planographic printing method in which the image areas and the non-image areas exist on almost the same plane, and the image areas and the non-image areas act as in acceptable layer and ink repellent layer, respectively. The ink adheres only to the image areas because of the difference in the adhesive properties, and the ink adhered to the image area is transferred to printing material such as paper. The feature of this method is to be able to perform printing without using a dampening solution.
There are various exposure methods proposed for waterless planographic printing plate precursors. They are broadly divided into two types: methods that use ultraviolet rays for irradiation through a plate making film, and computer-to-plate (hereinafter called “CTP”) methods in which the original pattern is directly incised without using a plate making film. The CTP methods are further divided into some types such as methods that use laser for irradiation, those using a thermal head for image transfer, those using a pin electrode to apply voltage to specific parts, and those using an ink-jet apparatus to form an ink acceptable layer or an ink repellent layer. Of these, methods that use laser irradiation are superior to the other methods in terms of resolution and plate processing speed.
There are two types of methods that use laser irradiation: the photon mode that uses photoreaction and the heat mode that uses photothermal conversion to cause thermal reaction. The utility of the heat mode is increasing because of its advantage for use in a bright room and the rapid progress of the semiconductor laser to be used as light source.
Various proposals have been made relating to directly imageable waterless planographic printing plate precursors that is designed for the heat mode described above. In particular, directly imageable waterless planographic printing plate precursors that contain bubbles in their heat sensitive layers have been proposed as a type of directly imageable waterless planographic printing plate precursors that can perform plate processing with less laser radiation energy and achieve a high image reproducibility (see Japanese Unexamined Patent Publication (Kokai) No. 2005-300586 (Claims), for example). To produce a directly imageable waterless planographic printing plate precursor that can perform plate processing with less laser irradiation energy and achieve a high image reproducibility, proposals have been made for directly imageable waterless planographic printing plate precursor production methods that comprise a step of applying a solution of a heat sensitive layer composition containing an organic solvent with a solubility parameter of 17.0 (MPa)1/2 or less and a step of drying the heat sensitive layer composition (see Japanese Unexamined Patent Publication (Kokai) No. 2005-33192 (Claims), for example).
The directly imageable waterless planographic printing plate precursors produced by the technology described in Japanese Unexamined Patent Publication (Kokai) No. 2005-300586 (Claims) and Japanese Unexamined Patent Publication (Kokai) No. 2005-331924 (Claims) are highly sensitive immediately after the precursor making process, but there has been the problem of sensitivity degradation over time. Thus, it could be helpful to provide directly imageable waterless planographic printing plate precursors that are highly sensitive not only immediately after the precursor making process but also after the passage of time.